Wet strength paper comprising starch carbamoylethyl ethers



United States Patent 3,542,644 WET STRENGTH PAPER COMPRISING STARCH CARBAMOYLETHYL ETI-IERS Herbert E. Smith, Sherald H. Gordon, and Herbert C. Katz, Peoria, Ill., assignors to the United States of America as represented by the Secretary of Agriculture No Drawing. Filed Aug. 9, 1968, Ser. No. 751,341

Int. Cl. D2lh 3/28 US. Cl. 162-175 1 Claim ABSTRACT OF THE DISCLOSURE Papers prepared from pulps containing 5 percent additions of carbamoylethyl ethers of starch or wheat flour that have then been crosslinked with sodium hypochlorite exhibit greatly increased wet tensile strength accompanied by less spectacular but significantly. improved dry tensile and burst values.

A nonexclusive, irrevocable, royalty-free license in the invention herein described, throughout the world for all purposes of the United States Government, with the power to grant sublicenses for such purposes is hereby granted to the Government of the United States of America.

BACKGROUND OF THE INVENTION This invention relates to the discovery that a wet-end incorporation in a papermakers cellulosic pulp of a hypochlorite-crosslinked starch carbamoylethyl ether having a carbamoylethyl D.S. of 0.25--0.43 increases the wet strength of the paper by a factor of about -14 while less conspicuously but appreciably also increasing the dry strength properties thereof.

The consumer benefits of improving the wet strength of such paper products as towels, toilet tissue, grocery bags, etc., during the relatively few years since the patenting by this laboratory of wet strength paper containing dialdehyde starch has lead to widespread industrial research and development in this field.

A diversity of wet strength resins and cationic starches that improve the wet strength of paper are now known, but many of these products share the still excessive cost limitation of dialdehyde starch. Furthermore, the resins that provide the largest increases in wet strength generally fail to also improve the dry strength properties of the paper, or they even embrittle the paper and thereby actually decrease the dry strength values of the paper.

The object of the instant invention is the provision of a process whereby the incorporation of a critically precrosslinked novel starch derivative in papermakers cellulosic pulps provides low cost papers that exhibit greatly increased wet strength properties as well as lesser but significantly increased dry strength values.

In accordance with the broad object of the invention we have now discovered that wet-end incorporations at a level of about 5 percent of an ex situ, i.e., precrosslinked starch carbamoylethyl ether having a carbamoylethyl D.S. of 0.250.43 or of a similarly derivatized and crosslinked Wheat flour provides unpredictably great increases, e.g.,

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0.02 to about 0.27 are added to paper pulps at levels of 0.5-1.0 percent as retention aids for pigments. The patent also suggests only retention aid employment for the allegedly crosslinked starch carbamoyl ethers produced by reacting starch with an acrylamide-formaldehyde mixture, or with either N-methylol acrylamide or N,N'-methylene bisacrylamide, any crosslinks whereof would necessarily be exclusively through methylene groups. Even if the methylene crosslinked species of Cushings starch ethers were capable of markedly increasing the wet strength of paper, which we have conclusively found they are not, the low levels of addition used by Cushing would at most provide only a nominal increase in wet strength that could not possibly have alerted Cushing to the highly unobvious wet-strength improvements of the instant invention wherein applicants have also found that only the starch carbamoylethyl ethers that have been critically crosslinked at about pH 2 with a very limited amount of a chlorine source such as a solution of commercial sodi um hypochlorite are operative as wet-strength additives, whereas the same starch ethers wherein crosslinking was attempted with hydrogen peroxide, periodic acid, nitrous acid, bromine water, or with iodine were inoperative for applicants purpose. Applicants believe this apparent inconsistency may be explainable on the basis that starch ethers containing carbamoylethyl groups react with the available chlorine of the hypochlorite solution to form a minor proportion of the corresponding N-chloro amide groups which groups in turn each react with a hydrogen atom of an unchanged carbamoylethyl group to eliminate a molecule of HCl and forms an imide-type crosslink between two nitrogen atoms.

Inaccordance with the general object of our invention We have now unexpectedly discovered that paper exhibiting tremendously increased wet strength and modestly increased dry strength is obtained from cellulosic pulps containing 5 percent based on the dry weight of the pulp of a sodium hypochlorite crosslinked carbamoylethyl ether of starch wherein the carbamoylethyl D.S. is 0.25-0.43.

The following illustrative embodiment and tables will enable those skilled in the art to directly practice our invention in the most effective manner. All quantlties are in parts by weight unless otherwise noted.

EXAMPLE 1 An ungelatinized starch carbamoylethyl ether containing 3.12 g. nitrogen and having a carbamoyl ether D.S. of 0.433 was prepared by slurrying 162 g. (dry basis) of pearl corn starch in 234 ml. of saturated sodium sulfate solution, and then to the slurry adding in turn 60 g. of acrylamide and, by dropwise addition, 17.5 g. of 24 percent NaOH solution. The constantly stirred mixture was reacted at 40-42 C. for 18 hours, diluted to a volume of ca. 1200 ml. with distilled water, the pH adjusted to 6.8 with 1 N HCl, and the crude starch either was isolated by filtration. A portion of the crude material was purified by washing several times with water, partially dehydrated with absolute ethanol, vacuum dried over H and ground to a 40 mesh powder, the last two steps also being applied to the unpurified portion.

A 1 percent starch ether paste or dispersion was prepared by weighing 5 g. of either of the above powders into a 3-necked flask equipped with a stirrer and a thermometer housed in a mercury well. Following the addition of 495 g. tap water, the stirred suspension was heated on a steam bath for 30 minutes at C. in a closed system, and then was allowed to cool.

To prepare experimental handsheets, 30 g. of wet unbleached kraft pulp containing 7.2 g. of solids (dry basis) was diluted with 293 g. tap water. Then 36.0 g. of the 1 percent starch ether paste (corresponding to a 5 percent level of addition) was weighed into a beaker, the pH was lowered to 2 with 0.5 ml. 6 N H 80 and the starch ether was crosslinked by the addition of 1 ml. of commercial sodium hypochlorite solution containing 5-5.2 percent of available chlorine. The visibly flocculated 4 EXAMPLE 3 Example 1 was repeated with the exceptions that the refined starch carbamoylethyl ether additive from the reaction of 1 mole (162 g.) of starch and 40 g. (0.56

crosslinked starch ether was then added to the dispersed 5 mole). acrylamide and analyzing percent corre' pulp and thoroughly blended therewith. The resulting sPondmg to carbamoylfithyl of 031?, was cross p p fu ish, avi g an v dry c n istency f 2 perc llnked as previously described and then substltuted for the was divided into 6 equal parts and each part upon trans starch ether having a of 0.433. Handsheets prepared fer to the handsheet mold was diluted with 99 parts of 10 from a l j Contammg f 5 P t by welght of dry tap Water to give a final pulp consistency of 002 percent, filter addition of the crosslinked 0.316 D.S. starch ether from which a standard handsheet was formed. The rep had a value of a tensile of 3 and licate handsheets were oven dried for 15 minutes at tenslle of 3790 compared Wlth correspondmg Values 105 C. and equilibrated for 7 days in a controlled 9 9 and for handsheets made from an humidity room before testing them and pertinent com 15 identical furnish exceptlng that the starch ether additive trols by standard TAPPI methods (T-4040S61) to obhad been cmsshnkedtain the data presented in Table I, below, which also sets EXAMPLE 4 forth comparisons with handsheet controls lncludlng the noncrosslinked satrch ether at several levels of addition Wheat flour, 162 g. dry basis, analyzing 2.44 percent and comparison data for handsheets containing the prior N corresponding to a protein content of 13.9 percent was art N-methylolacrylamide starch ethers. Also, handsheets substituted for the starch reactant of Example 3. The of the invention that were slowly dried in air at room temthusly produced crude wheat flour carbamoylethyl perature instead of in an oven exhibited somewhat lowether derivative after being crosslinked at pH 2 with 1 er increases in wet and dry strengths, ml. of NaOCl SOllltiOn was added at a level of 5 percent TABLE I Percent Breaking length, addltion Basis Burst, meters of starch NaOCl, wt., g. cm. Description of furnish ether n11. g.s.m. g.s.1n. Dry Wet Pulp control 61.4 43.3 6,770 240 Pulp plus uncrosslinked 0.433 D.S. starch ether 2 64. 3 56. 6 7,810 290 Pulp plus crosslinked 0.433 D.S. starch ether 2 0. 2 61.8 73. 1) 11,440 3,120 D01 2 0.4 01.3 75.7 11,420 3,370 D01 2 0.6 02.0 75.0 10, 050 3,040 Pulp plus uncrosslinked 0.433 D.S. starch ether 1 5 64. 4 58.4 9, 110 330 Pulp plus crosslinked 0.433 D S. starch ether 1 5 0. 5 61. 0 81. 6 11,480 3, 870 150. 5 1.0 61.1 80.0 11,790 4,010 130. 5 1.5 65.1 70.2 11,580 4,050 Pulp plus uncrosslinked N-methylolacrylannde starch ether; percent N=.05; D.S.=.003. 5 58. 4 62. 6 9, 606 310 Pulp plus crosslinked N-rnethylolacrylamide starch ether; percent N=.05; D.S. =.009 5 1. 0 60.0 63. 6 9, 620 860 Pulp plus uncrosslinked N-methylolaerylamidstarch etehr; percent N=1.0; D.S.=.125 55 "1.6-

Pulp plus crosslinked N-methylolacrylamide starch ether; percent N =1.0; D.S. =.125

1 Refined as shown in Table II, the infra nonrefined starch ether additives provide nearly as much strength improvement and probably would be prcicrrcd [or their somewhat lower cost.

EXAMPLE 2 Using 31 g. (0.43 mole) of acrylamide instead of 60 g. as in Example 1, a crude, i.e., washed carbamoylethyl ether of starch was obtained having a nitrogen content of 1.98 percent, representing a carbamoylethyl, D.S. of 0.257, which value was decreased slightly to a calculated carbamoylethyl D.S. of 0.250 in the refined half of the product.

A larger batch of cellulosic pulp identical to that used in Example 1 was prepared, and handsheets were prepared after appropriate portions thereof were fortified by the addition of the above crude or purified starch carbamoylethyl ether per se or the starch ether it had been after being crosslinked with sodium hypochlorite solution at pH 2. The strength values of the handsheets prepared therefrom are given in Table H.

Example 2 was repeated with the exceptions that a bleached kraft softwood pulp having a freeness of 390 was substituted for the unbleached kraft pulp, and employing the crude, i.e., unwashed, hypochlorite crosslinked starch ether having a carbamoylethyl D.S. of 0.257. The furnish fortified with the crosslinked starch TABLE 11 Percent addition of above starch Breaking length, ether or Basis Burst, meters compari- NaOCl wt., g. cm. Description of furnish son agent ml. g.s.m. gAm. Dry Wet Pulp control 61. 0 45.0 7, 060 226 Pulp plus NaOl, control- 0.4 62. 8 43. 3 065 255 Native unmodified starch 2 63. 2 53. 3 7, 340 189 Do 2 0. 4 65. 5 54. 5 7, 890 310 Pulp plus uncrosslinked refined 1 starch ether- 2 63. 2 45. 6 7, 580 240 Pulp plus crosslinked refined 1 starch ether 2 0. 4 61. 1 78. 2 11, 3, 350 Pulp plus crosslinked crude 2 starch ether- 2 4 58. 6 78. 7 11, 150 2, 990 Pulp plus NaOOl, controL 62.4 43.8 7, 545 380 Pulp plus native unmodified starch 5 64. 7 58. 7 8, 216 Pulp plus native unmodified starch plus NaOCL- 5 1. 0 63. 5 56. 7 7, 990 470 Pulp plus uncrosslinked refined 1 starch ether.. 6 63. 4 46. 5 7, 960 390 Pulp plus crosslinked refined 1 starch ether 5 1. 0 65. 5 83. 7 12, 350 3, 990 Pulp plus crosslinked crude 2 starch ether 5 1. 0 57. 2 76. 0 11, 080 3, 420

1 Carbamoylethyl D.S. 0.250. 3 Oarbamoylethyl D.S. 0.257.

ether at a 5 percent level of addition provided handsheets that had a burst strength of 71.8, a dry tensile value of 11,210, and a wet tensile of 2,250 compared with values of 60.1, 9,580, and 240 for handsheets from the furnish containing 5 percent of the fiber weight of the noncrosslinked otherwise identical starch ether, also compared with control values of 58.5, 8,980, and 230 for sheets from the unfortified bleached pulp.

We claim:

1. A process for producing kraft papers having a TAPPI wet tensile value of at least 3,000 meters along with materially improved dry tensile and burst strengths comprising dispersing into a papermakers cellulosic pulp slurry from 2 to 5 percent, based on the dry weight of cellulosic fibers, of a sodium hypochlorite-crosslinked starch carbamoylethyl ether wherein the crosslinking was 6 efiected at a pH of about 2, having a canbamoylethyl D5.

of 0.25-0.43, forming a wet web therefrom and oven drying the Web to form paper.

References Cited UNITED STATES PATENTS 3,033,852 5/1962 Paschall 260233.3

3,135,738 6/1964 Cushing 260-2333 3,472,671 10/1969 Wilkinson et a1. 260233.3 X

S. LEON BASHORE, Primary Examiner F. FREI, Assistant Examiner US. Cl. X.R. 260233.3 

